Uroguanylin, guanylin and bacterial ST peptides are structurally related peptides that bind to a guanylate cyclase receptor and stimulate intracellular production of cyclic guanosine monophosphate (cGMP) (1-6). This results in the activation of the cystic fibrosis transmembrane conductance regulator (CFTR), an apical membrane channel for efflux of chloride from enterocytes lining the intestinal tract (1-6). Activation of CFTR and the subsequent enhancement of transepithelial secretion of chloride lead to stimulation of sodium and water secretion into the intestinal lumen. Therefore, by serving as paracrine regulators of CFTR activity, cGMP receptor agonists regulate fluid and electrolyte transport in the GI tract (1-6; U.S. Pat. No. 5,489,670). Thus, the cGMP-mediated activation of CFTR and the downstream signaling plays an important role in normal functioning of gut physiology. Therefore, any abnormality in this process could potentially lead to gastrointestinal disorders such as irritable bowel syndrome, inflammatory bowel disease, excessive acidity and cancer (25, 26).
The process of epithelial renewal involves the proliferation, migration, differentiation, senescence, and eventual loss of GI cells in the lumen (7, 8). The GI mucosa can be divided into three distinct zones based on the proliferation index of epithelial cells. One of these zones, the proliferative zone, consists of undifferentiated stem cells responsible for providing a constant source of new cells. The stem cells migrate upward toward the lumen to which they are extruded. As they migrate, the cells lose their capacity to divide and become differentiated for carrying out specialized functions of the GI mucosa (9). Renewal of GI mucosa is very rapid with complete turnover occurring within a 24-48 hour period (9). During this process mutated and unwanted cells are replenished with new cells. Hence, homeostasis of the GI mucosa is regulated by continual maintenance of the balance between proliferation and apoptotic rates (8).
The rates of cell proliferation and apoptosis in the gut epithelium can be increased or decreased in a wide variety of different circumstances, e.g., in response to physiological stimuli such as aging, inflammatory signals, hormones, peptides, growth factors, chemicals and dietary habits. In addition, an enhanced proliferation rate is frequently associated with a reduction in turnover time and an expansion of the proliferative zone (10). The proliferation index has been observed to be much higher in pathological cases of ulcerative colitis and other GI disorders (11). Thus, intestinal hyperplasia is the major promoter of gastrointestinal inflammation and carcinogenesis.
In addition to a role for uroguanylin and guanylin as modulators of intestinal fluid and ion secretion, these peptides may also be involved in the continual renewal of GI mucosa by maintaining the balance between proliferation and apoptosis in cells lining GI mucosa. Therefore, any disruption in this renewal process, due to reduced production of uroguanylin and/or guanylin can lead to GI inflammation and cancer (25, 26). This is consistent with previously published data in WO 01/25266, which suggest a peptide with the active domain of uroguanylin may function as an inhibitor of polyp development in the colon and may constitute a treatment of colon cancer. However, recent data also suggest that uroguanylin also binds to a currently unknown receptor, which is distinct from GC-C receptor (3,4). Knockout mice lacking this guanylate cyclase receptor show resistance to ST peptides in the intestine, but effects of uroguanylin and ST peptides are not disturbed in the kidney in vivo (3). These results were further supported by the fact that membrane depolarization induced by guanylin was blocked by genistein, a tyrosine kinase inhibitor, whereas hyperpolarization induced by uroguanylin was not effected (12, 13). Thus, it is not clear if the anti-colon cancer and anti-inflammatory activities of uroguanylin and its analogs are mediated through binding to one or both of these receptors.
Inflammatory bowel disease is a general name given to a group of disorders that cause intestines to become inflamed, characterized by red and swollen tissue. Gastrointestinal (GI) inflammation can be a chronic condition and often leads to GI cancer (14). Examples of such inflammatory bowel diseases (IBD) include Crohn's disease and ulcerative colitis (UC). It is estimated that as many as 1,000,000 Americans are afflicted with IBD, with male and female patients appearing to be equally affected. Most cases are diagnosed before age 30, but the disease can occur in the sixth, seventh, and later decades of life.
Crohn's disease is a serious inflammatory disease that predominantly effects ileum and colon, but can also occur in other sections of the GI tract, whereas UC is exclusively an inflammatory disease of the colon, the large intestine (15). Unlike Crohn's disease, in which all layers of the intestine are involved, and in which there can be normal healthy bowel in between patches of diseased bowel, UC affects only the innermost lining (mucosa) of the colon in a continuous manner (16). Depending on which portion of the GI tract is involved, Crohn's disease may be referred to as ileitis, regional enteritis, colitis, etc. Crohn's disease and UC differ from spastic colon or irritable bowel syndrome, which are motility disorders of the GI tract.
While the precise cause of IBD is not known, it is believed that the disruption of the process of continual renewal of GI mucosa may be involved in disease (17,18). The renewal process of the GI lining is an efficient and dynamic process involving the continual proliferation and replenishment of unwanted damaged cells. Proliferation rates of cells lining the GI mucosa are very high, second only to the hematopoietic system. Thus, the balance between proliferation and apoptosis is important to the maintenance of the homeostasis of the GI mucosa (19,20).
GI homeostasis depends on both proliferation and programmed cellular death (apoptosis) of epithelial cells lining the gut mucosa. Hence, cells are continually lost from the villus into the lumen of the gut and are replenished at a substantially equal rate by the proliferation of cells in the crypts, followed by their upward movement to the villus. It has become increasingly apparent that the control of cell death is an equally, if not more, important regulator of cell number and proliferation index (19,20). Reduced rates of apoptosis are often associated with abnormal growth, inflammation, and neoplastic transformation. Thus, both decreased proliferation and/or increased cell death may reduce cell number, whereas increased proliferation and/or reduced cell death may increase the proliferation index of intestinal tissue (20), which may lead to GI inflammatory diseases and cancer.
Uroguanylin and guanylin peptides also appear to promote apoptosis by controlling cellular ion flux. Alterations in apoptosis have been associated with tumor progression to the metastatic phenotype. While a primary gastrointestinal (GI) cancer is limited to the small intestine, colon, and rectum, it may metastasize and spread to such localities as bone, lymph nodes, liver, lung, peritoneum, ovaries, and brain. By enhancing the efflux of K+ and influx of Ca++, uroguanylin and related peptides may promote the death of transformed cells and thereby inhibit metastasis
Irritable bowel syndrome (IBS) and chronic idiopathic constipation are pathological conditions that can cause a great deal of intestinal discomfort and distress but unlike the IBD diseases such as ulcerative colitis and Crohn's disease, IBS does not cause the serious inflammation or changes in bowel tissue and it is not thought to increase the risk of colorectal cancer. In the past, inflammatory bowel disease (IBD), celiac disease and irritable bowel syndrome (IBS) were regarded as completely separate disorders. Now, with the description of inflammation, albeit low-grade, in IBS, and of symptom overlap between IBS and celiac disease, this contention has come under question. Acute bacterial gastroenteritis is the strongest risk factor identified to date for the subsequent development of postinfective irritable bowel syndrome. Clinical risk factors include prolonged acute illness and the absence of vomiting. A genetically determined susceptibility to inflammatory stimuli may also be a risk factor for irritable bowel syndrome. The underlying pathophysiology indicates increased intestinal permeability and low-grade inflammation, as well as altered motility and visceral sensitivity (27). Serotonin (5-hydroxytryptamine[5-HT]) is a key modulator of gut function and is known to play a major role in pathophysiology of IBS. It has been shown that the activity of 5-HT is regulated by cGMP (28). Therefore, based on this observation as well as other effects of cGMP, we believe that GC-C agonists will be useful in the treatment of IBS.
Given the prevalence of inflammatory conditions in Western societies and the attendant risk of developing cancerous lesions from inflamed tissue, particularly intestinal tissue, a need exists to improve the treatment options for inflammatory conditions, particularly of the gastrointestinal tract.